1. Field of the Invention
The present invention relates to a light-emitting device or a lighting device which emits light by electroluminescence.
2. Description of the Related Art
In recent years, a light-emitting element in which a light-emitting organic compound or a light-emitting inorganic compound is used as a light-emitting substance has been actively developed. In particular, a light-emitting element called an electroluminescent (EL) element has a simple structure in which a light-emitting layer containing a light-emitting substance is provided between electrodes and has the following characteristics: its thickness and weight can be reduced; it can respond to input signals fast; and it can be driven at low DC voltage.
Such a light-emitting element is expected to be applied mainly to a display and lighting. As applications to lighting, there are conventional lighting devices such as a filament lamp and a fluorescent lamp which have a point light source and a linear light source, respectively. In contrast, the light-emitting element can provide surface light emission; thus, it is considered that the light-emitting element can be used for manufacturing a lighting device with a nonconventional shape, such as a sheet-like lighting device. Moreover, the light-emitting element has a surface light source; thus, lighting closer to natural light can be easily obtained.
Light-emitting elements utilizing EL emission (the light-emitting element is hereinafter referred to as an EL light-emitting element) are classified, according to a substance contained in a light-emitting layer, into a light-emitting element utilizing emission generated in returning from a singlet excited state to a ground state (i.e., fluorescence) and a light-emitting element utilizing emission generated in returning from a triplet excited state to the ground state (i.e., phosphorescence). Only 25% of the total excitons can be generated as excitons in the single excited state in the EL light-emitting element; thus, examination of light-emitting substances which can utilize the triplet excited state has been actively conducted in order to effectively utilize input electric power.
As a method of increasing power efficiency, reducing driving voltage of an EL light-emitting element is considered for the reason below. In the case where the same amount of current is applied to EL light-emitting elements, the EL light-emitting element to which the current is applied at lower voltage consumes less power than the other EL light-emitting element to which current is applied at higher voltage. However, in general, the luminance of an EL light-emitting element which can be driven at low voltage is significantly changed due to a small change in driving voltage.
In order to obtain sufficient luminous flux from an EL light-emitting element so that the EL light-emitting element is used for lighting, a method of increasing the emission area of the EL light-emitting element is convenient. However, increasing the emission area causes concern of variation in luminance of a light-emitting surface caused by voltage drop of a transparent thin electrode. In particular, an EL light-emitting element which can be driven at low voltage is strongly affected by the voltage drop, and therefore needs countermeasures against the voltage drop.
As the countermeasure, a structure in which a low resistant auxiliary wiring is formed over a transparent electrode has been reported (see Patent Document 1). This method, in which the low resistant auxiliary wiring is formed over the transparent electrode and an inorganic insulating layer is formed over only the auxiliary wiring, allows uniform light emission over the surface of the light-emitting element and a reduction in reactive power over the auxiliary wiring.    [Patent Document 1] Japanese Published Patent Application No. H11-97183